The two basic categories of coating application systems are excess coat and wipe systems and pre-metered systems, which differ in their method of controlling the amount of coating solution applied to a substrate. In excess coating application systems, an amount of solution in excess of the desired coating weight is applied to the substrate. A scraping device then removes the excess coating material from the substrate to achieve the desired coating weight. In a pre-metered application system, the amount of coating material is accurately measured and initially applied to the substrate to achieve the desired coating weight and does not require the removal of excess coating material.
Excess coating application systems can apply coating material to the substrate by a number of different techniques. For example, the substrate may be fed through a trough containing the coating solution so that the entire substrate is immersed in the coating solution. When the substrate exits the trough, a pair of rotating rollers are spaced appropriately from each other to wipe the excess coating material from the substrate. This removed coating material is typically recirculated into the system for return to the coating trough. For another example, blade or gap methods may be used, where a thick solution is allowed to flow through an adjustable opening directly onto a substrate, after which the excess solution is removed from the substrate surface. Other examples include dripping or forcing the coating solution onto the substrate, spraying the solution onto the substrate, and pressure or extrusion coating the solution onto the substrate. In any of these systems, devices such as knives, blades, rods, or rollers are used to remove the excess material that has been deposited onto the substrate, and the excess material is typically recirculated into the coating application system. Although excess coating application systems are useful for many types of substrates and coating materials, pre-metered coating application systems may be more desirable for certain types of coating solutions, coating weights, and substrates.
Pre-metered coating application systems include various types of coating equipment. In these systems;, knives or blades are typically used for smoothing a metered amount of coating material after it is applied to the substrate surface. In addition, these systems typically do not provide for recirculation of the coating solution since none of the solution applied to the substrate is removed from the substrate surface. Various slot coater configurations are described in references such as Coating and Laminating Machines (H. L. Weiss, Converting Technology Company 1977), which discusses different methods of foaming polymeric materials. One particular system for applying fluid to substrates includes a slot orifice coater, which is a coater that extrudes a solution through a gap and applies that extruded solution directly from the gap onto a moving substrate. In order to achieve a uniform distribution of fluid across the width of the substrate, it is necessary for the pressure within the coater to be relatively constant across the width of the coater. Therefore, slot coaters of this type are typically used with fluids having high viscosities and fluids being coated at high coating weights. Under these conditions, it is relatively easy to maintain uniform coating across the web. When these coaters are used with lower viscosity fluids and/or lower coating weights, it becomes more difficult to maintain uniform flow velocity and uniform hydrostatic pressure across the width of the slot.
In some slot coaters, the hydrostatic pressure of the fluid within the coater can actually build to the point where some portions of the coater will deflect and deform relative to other portions of the coater. If this happens, the slot height can vary across the width of the coater, thereby causing a nonuniform distribution of fluid to be coated onto the substrate. In order to achieve more uniform coatings, some slot coaters are thus provided with a choker bar or resistor bar assembly mounted outside the slot, which is a movable assembly that can be adjusted to change the gap height across the width of the coater. However, in order to accommodate the addition of an adjustment bar assembly, coaters with adjustment bar assemblies typically must be larger, sturdier, and consequently more costly and significantly more heavy and difficult to handle than a coater without an adjustment bar assembly. In addition, although these coater bars can correct some of the gap variations caused by high pressure, proper adjustment of the choker bar or resistor bar can typically only be accomplished by an operator who has considerable skill and experience adjusting the gap for a particular slot coater. To measure or monitor the effects of these adjustments, additional on-line measuring devices may also be required, which can be costly.
One method used to eliminate the requirements of gap adjustment is with a coating die known as a round multiple orifice (RMO) die of the type described in U.S. Pat. Nos. 4,391,856 (McIntyre et al.) and 5,264,036 (Hoechst et al.). An RMO die typically has a central reservoir from which multiple liquid nozzles extend. These nozzles terminate at the surface of the substrate so that the fluid is forced through the reservoir and nozzles, depositing the fluid directly onto the substrate surface in a series of fluid beads. A shear surface is oriented adjacent to the exit of these nozzles to merge the series of fluid beads together after the fluid is deposited onto the substrate. However, several parameters effect the uniformity of the merging of fluid beads across the substrate, such as the nozzle size, nozzle spacing, fluid viscosity, fluid quantity, and substrate porosity. As these parameters change, it can become more difficult for the shear surface to merge the beads together and the fluid will be coated onto the substrate in distinct stripes, rather than a continuous coating layer. For example, it is more likely that beads of high viscosity fluids will result in stripes along the length of the web than with beads of low viscosity fluids. However, as with the slot coaters, when RMO dies are used with lower viscosity fluids or lower coating weights, it becomes difficult to maintain uniform flow velocity and uniform pressure across the width of the die. Again, this problem is further emphasized when the width of the coater is increased.